I've seen conflicting conversions, americans and japanese measure these things differently (US uses maximum grain size, Japan uses average grain size or something like that). I've seen the norton 8k grit hone described as 3 microns, and Shapton claims their 15k hone is 0.96 micron. I've seen apparently identical chromium oxide pastes described as 0.5 micron, 30k grit, and 60k grit. Both Norton and Shapton make 8k water hones, but the 8k shapton seems noticeably finer than the norton; if the shapton is 8k I'd guess the norton is about 6k. What either of them really are is anybody's guess.

I'm not sure the exact numbers are that critical anyway. All you need is a progression you're comfortable working with.